Thermofluid
Questions & Answers
8) If the pressure, Px, is 75 kPa, find the pressure, Py, in the adjoining water pipe.
10) A 2500 L rigid tank contains 50 kg of air at a temperature of 50°C. Heat is added until the pressure is doubled. What is the final temperature inside the tank?
11) 3 kg or steam initially at 375 kPa and 141.3°C is contained in a piston-cylinder with a diameter of 88 cm. A spring, that is attached to the piston cylinder and initially at equilibrium at a piston height of 160 cm from the bottom, has a spring constant of 128,450N/m. If the piston raises to double its initial height, find the heat input into the steam.
The interior space of a building in winter is to be heated by a heat pump to maintain the internal temperature at 22.0°C. The 55.0 kW building heat load is to be provided by a heat pump that absorbs heat from a geothermal water source at 18.0°C. The water from the geothermal water source enters the evaporator heat exchanger at 18.0°C and exits at 11.00°C. The heat pump utilises refrigerant R-134a as the working fluid and operates with a discharge pressure of 1.40 MPa and a suction pressure of 320 kPa. Modelling the heat pump cycle as an actual vapour compression cycle (AVCC), with an evaporator exit temperature of 10.00°C and condenser entry and exit temperatures of 80°C and 30°C respectively, determine: What is the COP of this AVCC system? b. Determine mass flowrate of refrigerant (in kg/s). Find the required volumetric flowrate of water from the geothermal source (in L/min) d. Determine the isentropic efficiency of the compressor (assume the compressor to be adiabatic). e. If the heat pump were replaced by an ideal Carnot heat pump what would be the required power input to the heat pump? (Assume the heat load is still 55.0 kW).
1- Superheated steam at 1.5 MPa (15 bar), 600 °C enters a well-insulated turbine. The exit pressure is 70 kPa (0.7 bar). The turbine produces 10 MW of power. If the exit pipe is 1.6 m in diameter and carries 11 kg/s of flow, find the velocity at the exit. Neglect kinetic energy.
2- Consider natural gas with a molecular weight of 23.6 kg/kmol and a specific heat, c, of 2.01 kJ/kg K. The gas is slowly compressed in a frictionless, adiabatic process from an initial volume of 212 cm? to a final volume of 98 cm?. If the initial pressure is 39 kPa, and the initial temperature is 15 °C, find the final temperature and pressure. Assume the mixture can be modeled as an ideal gas.
1) The superheated water vapor is at 15 MPa and 350°C. The gas constant, the critical pressure, and the critical temperature of water are R = 0.4615 kPa m³/kg-K, Tcr= 647.1 K, and Pcr= 22.06 MPa. Use data from the steam tables. a) Determine the specific volume of superheated water based on the ideal-gas equation. b) Determine the specific volume of superheated water based on the generalized compressibility chart. c) Determine the specific volume of superheated water based on data from tables. d) Determine the error involved in the first two cases (a and b).
2) Air is compressed by an adiabatic compressor from 95 kPa and 27°C to 600 kPa and 277°C. Assume variable specific heats and neglect the changes in kinetic and potential energies. a) Determine the isentropic efficiency of the compressor. b) Determine the exit temperature of air if the process were reversible.
3) A steam turbine operates with 1.6 MPa and 350°C steam at its inlet and saturated vapor at 30°C at its exit. The mass flow rate of the steam is 21.8 kg/s, and the turbine produces 12,350 kW of power. Determine the rate at which heat is lost through the casing of this turbine.
4) It is commonly recommended that hot foods be cooled first to room temperature by simply waiting a while before they are put into the refrigerator to save energy. Despite this common-sense recommendation, a person keeps cooking a large pan of stew three times a week and putting the pan into the refrigerator while it is still hot, thinking that the money saved is probably too little. But he says he can be convinced if you can show that the money saved is significant. The average mass of the pan and its contents is 5 kg. The average temperature of the kitchen is 23°C, and the average temperature of the food is 95°C when it is taken off the stove. The refrigerated space is maintained at 3°C, and the average specific heat of the food and the pan can be taken to be 3.9 kJ/kg-°C. If the refrigerator has a coefficient of performance of 1.5 and the cost of electricity is $0.16/kWh, determine how much this person will save a year by waiting for the food to cool to room temperature before putting it into the refrigerator.
(2) Using example 1-5 as a guide, find Q1, Q2 and PB for L1= 1200 m P= 758 kPa L2=1200 m, D1-30 cm (0.3 m) ZA-36 m D2=20 cm (0.2 m), , 1=3x10 m2= 3x105m, ZB= 25 m. QA= 0.17 m³/s, p(density) = 1000 kg/m² v(kinematic viscosity)=10€ m²/s
(2) Using example 1-5 as a guide, find Q1, Q2 and PB for L1= 1200 m P= 758 kPa L2=1200 m. D1-30 cm (0.3 m) ZA-36 m D2=20 cm (0.2 m), 1=3x10 m 2= 3x105m, ZB= 25 m. QA= 0.17 m³/s, p(density)=1000 kg/m² v/kinematic viscosity)=10€ m²/s
You have been requested to determine if a volumetric flow of at least 0.025 m3/s can be achieved under the following scenarios. (1) Beginning of pumping cycle case with initial reservoir elevations (2) Beginning of pumping cycle case with a 15% safety factor added to the head loss (3) End of pumping cycle case with final reservoir elevations (do not include a 15% head loss safety factor) (4) Only using Pipe A (i.e., Pipe B is out of service) using one pump (5) Only using Pipe A (i.e., Pipe B is out of service) when operating both pumps in parallel (i.e., primary pump and redundant pump are both being using together)
in the figure below a piston is free to move and will not exit the cylinder. Initially the cylinder contains 2kg of air at 1.Sbar and 25°C Heat is transferred to the air and the piston rises until it reaches the blocks, at which point the volume is twice the initial volume. More heat is added until the pressure inside the cylinder also doubles. Determine i.the work dene and ii.the amount of heat transfer for this process. Also show the process on a P-v diagram. b) Air operating in a closed cycle consists of the following processes. Process 1-2: Reversible adiabatic compression. Process 2-3: Coristant volume heat addition. Process 3-4: Constant pressure heat addition. Process 4-5: Reversible adiabatic expansion. Process 5-1: Constant volume heat rejection. Draw this cycle on a pressure volume diagram and annotate your diagram with as much information as possible.
(a) A water tank is completely filled with liquid water at 60rc. The tank material is such that it can withstand tension caused by a volume expansion of 4%. Determine the maximum temperature rise allowed without jeopardizing safety. Take water coefficient expansion (B) as 5.22 x 10*1K at this temperature. (b) A weight, as shown in figure Qib-(a) has to move at constant velocity of 2 m's on an inelined surface with a coefficient of friction of 0.27. The width of the block is 20 cm. Determine the force (FI) that needs to be applied in the horizontal direction. By applying a thick oil film as shown in figure Qlb-b), the force required to push the block reduced by 45%. If dynamic viscosity of the oil is 12 cP,determine the oil layer thickness. (c) A gate with 2 m width is kacated under the water as shown in Figure Qlc. IF the force(F) required to held the gate is about 20 kN, determine the distance (dj of this force to
a) A fan is installed in a residential building to provide proper ventilation. This fan is connected to a duct with I1.6 em diameter and provides the average air velocity of 5mis. By regulation, the minimum fresh air requirement is specified to be 0.35 air changes per hour (ACH) which means 35% of the entire air contained in a room should be replaced by fresh outdoor air every hour. Determine: 1. The flow capacity of the fan in litres/min What the height of the residential building should be. b) A3 cm orifice plate is placed within a 4 em pipe in which methanel at 20 "C (SG =0.7884 and dynamic viscosity (p) 0.5857 eP) is flowing through. If the flow rate passing through the pipe is 3.1 litres per seconds, determine the pressure difference that must be measured around the orifice plate. The discharge coefficient of the orifice can-be calculated by: C_{d d}=0.5959+0.0312 \beta^{2.1}-0.184 \beta^{k}+\frac{91.71 \beta^{23}}{\mathrm{Re}^{0.25}} Where, B is the ratio of orifiee diameter to pipe diameter and Re is the Reynolds number. c) A nozzle is fastened to the end of a U-tube with dimensions as shown in Figure Q2c.The nozzle exhausts into atmospheric pressure at 100 kPa; neglect friction and compute the force exerted on the U-tube by the water. if the force required to hold the nozzle is 850 N, determine the inlet pressure. d) The drag coefficient in aircraft industry affected by some parameters which are the speed of plane (v). the plane length (L), the air density (pk. the air dynamite viscosity(p), and speed of sound (a). By using dimensional analysis, identify two non-dimension numbers in which the drag coefficient is a function of them and explain how these two will effect on drag coefficient.
a) Given that for a flew process the steady flow equation is: q-w=\left(u_{2}-u_{1}\right)+\left(p_{2} v_{2}-P_{1} v_{1}\right)+\left(\frac{C^{2}}{2}-\frac{C_{1}^{2}}{2}\right)+p\left(x_{2}-z_{1}\right) b) Explain how the following jet engine works. Refer to each section of the engine and explain the purpose of each section. e) Draw a Temperature (T) versus Entropy (s) diagram for a typical gas turbine. The diagram should show the processes for an actual gas turbine and should show the effect of real world losses where the compressor and turbine have an isentropic efficiency and pressure losses in the combustion chamber in an actual gas turbine.Explain each of the processes on the T-s diagram.
Question 2 You are to design a finned heat sink for a computer mother board, which comprises of vertically mounted rectangular parallel plates on a porous base, as shown in figure 2. The height of each fin is 40 mm, and the temperature of the fins are constant at 80 °C. The heat from the fins induces an air flow (due to free convection) from the surroundings through the porous base and through the parallel plates, where they exit at the top. The surrounding temperature is constant at 20 °C. The base is square, with a width of 50 mm. Neglect heat transfer Air flow (via free convection) Fins Neglect heat transfer 50 mm Porous base Neglect heat transfer Figure 2: Free convection in between parallel plates. 40 mm Neglecting the heat transfer from the bottom of the base, and through the extreme left and right side of the fins, i.e., the only heat transfer is due to the convection in between the parallel plates (as shown in figure 2), calculate the minimum number of fins (plates) required if the required heat transfer rate is 18 W. [10 marks] You may assume that the thickness of the fins is negligible.
Question 3 A large horizontal brass plate is used to boil water at atmospheric pressure. What is the maximum permitted temperature of the plate so that it does not exceed the critical heat flux? [8 marks]
This is an individual assignment, and is worth 30 marks in total. You will need to answer all questions, and you need to bear in mind that in some instances you may need to use knowledge you have previously gained in other courses to solve these questions. Please submit your assigment online. Typed solutions are recommended, but clearly presented hand-written solutions will also be accepted. This assignment is due on 8th October at 11pm. If you submit your assignment late, you will be deducted 4 marks (i.e. 20%) per day that you are late. Question 1 Sheets of steel are typically made by rolling heated steel at high temperatures, resulting in a flat plate as shown in figure 1. The resulting hot flat plates are cooled by passing air at a fixed temperature of 295 K over the plate. The plate is 4 m long, and 1.2 m wide, and has a uniform temperature of 600 K. The velocity of the flow of air is 9.5 m/s. Air 4 m Figure 1: Flow of air over a flat plate. Assuming the sheets are oriented such that the air flow along the length of the plate (i.e., along the a direction, as denoted by the blue arrow), calculate: a) The local heat transfer coefficient at a distance 1 m from the leading edge of the plate (i.e. at x = 1 m) [4 marks] b) The average heat transfer coefficient over the entire plate [3 marks] c) The rate of heat transfer from the plate to the air. [1 mark] If the plate is re-oriented such that the flow of air now flows along the width of the plate (i.e. along the y direction, as denoted by the red arrow), calculate the rate of heat transfer from the plate to the air. [4 marks] In all cases above, you may neglect any heat transfer from the bottom of the plate.
